1. Field of the Invention
The present invention relates to a PZT-based ferroelectric thin film, which can be used for a dielectric layer or the like of a thin film capacitor, and a method of forming the same. More specifically, the invention relates to a PZT-based ferroelectric thin film having superior electrical properties, long service life, and superior mass-productivity; and a method of forming the same.
Priority is claimed on Japanese Patent Application No. 2013-73150, filed on Mar. 29, 2013, the content of which is incorporated herein by reference.
2. Description of Related Art
When a ferroelectric thin film is formed using a sol-gel method, typically, a high-temperature process such as pre-baking or baking is performed. Therefore, when an attempt to obtain a thicker film is made by increasing the coating amount applied in each coating process, tensile stress generated in the film during baking or the like is increased, which may cause a problem of cracking in the formed film.
When cracking occurs in the formed film, the electrical properties or the like of the ferroelectric thin film deteriorate. In the related art, the thickness of a film which can be formed in each coating process using a sol-gel method is limited to about 100 nm. In order to form a thick ferroelectric thin film, a method of performing coating and baking processes of a composition multiple times is adopted. However, with this method, production efficiency decreases, which leads to an increase in film forming cost. Therefore, studies and developments have been actively made regarding improvement of a material, that is, regarding a raw material solution capable of increasing the thickness of a film formed in each coating process without cracking.
In addition, one of the issues during the formation of a ferroelectric thin film using a sol-gel method is to increase the service life of a thin film. For example, the document “Influence of Film Texture on Reliability of Sol-gel Derived PZT Thin-film Capacitors (T. Noguchi, et. al. Key Eng. Mater. In press (2013))” reports that, by laminating layers of a sol-gel solution to which a specific organic dopant is added, a PZT ferroelectric thin film having grown crystal grains can be formed, and the service life of the obtained thin film is longer than that of a thin film of the related art having fine crystal grains. Japanese Unexamined Patent Application, First Publication No. 2012-009800 (claim 1, paragraph [0012]) focuses on a structure of a ferroelectric thin film and discloses a ferroelectric thin film in which a microstructure is controlled to improve service life reliability. The ferroelectric thin film disclosed in Japanese Unexamined Patent Application, First Publication No. 2012-009800 has a configuration of a mixed composite metal oxide in which metal oxides B composed of Bi, Si, and the like are mixed with a PZT-based composite metal oxide A at a predetermined molar ratio. In this configuration, 2 to 23 baked layers are laminated, the thickness t of each of the baked layers is 45 nm to 500 nm, an average value x of maximum grain sizes of crystal grains present in each of the baked layers in a predetermined direction is 200 nm to 5000 nm, and all the baked layers satisfy a relationship of 1.5t<x<23t. In the ferroelectric thin film disclosed in Japanese Unexamined Patent Application, First Publication No. 2012-009800, the grain size of crystal grains present in each of the baked layers is increased to be greater than that of crystal grains present in a thin film prepared using a CSD method of the related art, and interfaces are introduced into the film by laminating a plurality of baked layers. As a result, the service life reliability of a thin film capacitor or the like is improved.